UK Smoke and Sulphur Dioxide Network

Local Air Quality Reviews and Assessments

7.1 STAGES OF THE REVIEW AND ASSESSMENT PROCESS
Data from the Smoke and SO₂ Network have always been widely used at a local level to help to define air quality, and are increasingly published in local authority and county environmental reports. One important potential use at the present time is in Local Authorities' air quality reviews and assessments, as required by Part IV of the Environment Act, 1995. Local Authorities are required to undertake a review and assessment of local air quality, with reference to the standards and objectives of the National Air Quality Strategy. There are three stages to the reviews, which must be carried out for all the pollutants covered by the Strategy (except ozone):
1. First stage review and assessment. This involves collating information on current and potential future sources of the pollutant, with a view to assessing whether the Strategy objectives are likely to be met by 2005. Pollutant Specific Guidance⁹ lists processes and activities for each pollutant, which, singly or together, may emit significant quantities of the pollutant of concern. If there are none within the Local Authority's area, they need not proceed to the second stage.
2. Second stage. The aim of the second stage review and assessment is to provide a further screening of pollutant concentration. The emphasis should be on locations where the first stage predicted highest pollutant levels. This involves an estimation of background concentrations; maps are available from the DETR's Air Quality Archive on the World Wide Web. Contributions from specific sources can then be added to these. Measurements from existing monitoring programmes can be used; it is at this stage that data from non-automatic networks can make an important contribution.
3. Third Stage. Authorities will need to conduct a third stage review and assessment if the first and second stages have indicated that there is a significant risk of air quality objectives not being achieved by the end of 2005.
Local Authorities are currently engaged in this review and assessment, following Pollutant Specific Guidance issued by the DETR⁹ . In the case of SO₂, many Authorities will have to proceed to the second or third stage, while for particulate matter, measured as PM₁₀, it is likely that almost all Authorities will have to proceed to at least a second stage review.

7.2 USE OF NON-AUTOMATIC SO₂ DATA IN REVIEW AND ASSESSMENT
For local authorities which have to proceed to the second stage for SO₂, data from Network or other sites can be of use. Daily data cannot of course be directly compared with the SO₂ objective for the annual 99.9th percentile of 15 minute means; however, the study on surrogate statistics⁸ discussed in section 6.1 has found that the air quality objective is unlikely to be exceeded if the maximum daily mean is less than 48 ppb. The relationship, which is only valid for SO₂, was determined on the basis of data from automatic monitors. If data from the Smoke and SO₂ Network is used, the maximum daily mean should be multiplied by a factor of 1.25, to take account of the tendency for the peroxide bubbler technique used in this Network to under-read at peak concentrations. A further point to note is that Network sites analyse their SO₂ data by acid titration, which measures total acidity rather than specifically SO₂. While not ideal, total acidity data from this network will still have a role to play in the second stage review and assessment for SO₂.

7.3 USE OF BLACK SMOKE DATA IN THE REVIEW AND ASSESSMENT
For several years, there has been considerable interest in the possibility of using black smoke data to model or predict concentrations of PM₁₀. However, there are considerable differences between these two methods of particulate measurement. While PM₁₀ is specifically defined as the mass fraction of particles collected by a sampler with a 50% cut-off at aerodynamic diameter 10 µm, the size fraction sampled by the black smoke method is not well defined but has a 50% cut-off between 3 and 5 µm ¹⁰. Black smoke is, therefore, likely to approximately represent "dark PM₄", and may be considered a subset of PM₁₀.

Work by Stedman ^11,12, discussed in last year's report and elsewhere, includes development of a method to assign contributions to PM₁₀ concentration to three sources;
- Primary particulate material, mostly form local combustion sources. Black smoke provides a good indicator of this component.
- Secondary material, mostly light coloured sulphate and nitrate particles. Measured sulphate concentrations, at the UK's network of eight rural sites, provides a good indicator of this component. It is important to note that nitrates may make a significant contribution to this material. However, sulphates do provide a good indicator, and are measured more widely than nitrates.
- Coarse material, such as pollen, sea salt and wind blown dust. This varies from day to day and site to site; typical annual means are in the range 7 to 11 µg m^-3.
From this work, AEA Technology have developed a regression model, relating daily PM₁₀ to measured daily black smoke and sulphate concentrations. The relationship is of the form

total PM₁₀ = A x smoke + B x sulphate + C

This was derived for six monitoring sites, for 1996, in cities where there was an automatic PM₁₀ monitor, and at least one black smoke site. The sulphate concentration was taken from the nearest rural monitoring site, or the average of the two nearest (there were eight operating during 1996): concentration of sulphate and other secondary particulate is expected to be relatively uniform, due to its long atmospheric lifetime. The sulphate coefficient, B, represents the factor required to convert sulphate to total secondary particulate, taking into account counter ions such as ammonium, and also other components such as nitrate. The smoke concentration was the average of all the sites in the city, and the smoke coefficient A is typically between 0.6 and 1.0 depending on the site, and may reflect the types of local primary particulate sources. The constant C, representing the concentration of coarser particulate within the PM₁₀ fraction, from wind-blown dust etc. is typically between 7 and 11 µg m^-3. These values are shown in Table 8, taken from the Airborne Particles Expert Group (APEG) Interim report on source apportionment¹³.

The technique has been extended to provide forecasts of PM₁₀ concentrations in the years 2005 and 2010. Both primary combustion related particulate and secondary particulate are predicted to decrease in future years, while coarse particulate levels are predicted to remain unchanged. Using the predicted changes in the various components, estimates can be made predicting future concentrations of total PM₁₀.

The APEG report¹³ does not specifically concentrate on the requirements of Local Authority Air Quality Review and Assessments. However, during 1997 there were 37 automatic PM₁₀ sites, whereas there were over 200 smoke and SO₂ Network sites. A useful application for this work is the potential for using daily measured black smoke data, together with daily measured sulphate data from the rural network, to predict PM₁₀ in towns with no automatic PM₁₀ monitoring, given sufficient additional information. The smoke coefficient for any given Local Authority area can be estimated by comparison with measurements from sites with automatic PM₁₀ monitoring, together with UK Emission Inventory data. The sulphate coefficient, B, can be estimated on a regional basis from interpolation of measurements at the network of rural sites. This work forms the basis of a model which can estimate PM₁₀ concentrations from black smoke data¹³.